The invention relates to the vacuum distillation of crude hydrocarbon streams.
The first step in any petroleum refinery is the separation of crude oil into various fractions by distillation, in a so-called crude distillation unit (CDU).
A CDU typically comprises several components. The core of the unit is the main atmospheric distillation column, where the primary fractionation of a crude oil takes place. Steam is introduced into the column's bottom part for stripping purposes. The unit further comprises a preflash vessel and a furnace, both located upstream of the distillation column. The preflash vessel is located upstream of the furnace of the CDU. The vapor leaving the preflash vessel is normally bypassing the furnace and is routed into the main distillation column. Below the feed inlet there is normally a ‘stripping’ zone where steam is used. This improves gasoil recovery. Several side stream strippers are further connected to the main distillation column for recovering various fractions, such as kerosene (kero stripper) and gasoil (gasoil stripper). A naphtha splitter is usually present for recovering the naphtha fraction, whilst a debutanizer is used for recovery of the C3 and C4 hydrocarbon gases. The reflux arranged at the top of the main distillation column usually comprises a condenser and a reflux drum. Finally, the residue leaving the bottom of the distillation column of the CDU, also known as the long residue (LR), usually is further processed in a high vacuum unit (HVU).
Typically, HVU's aim at separation of vacuum gasoil (VGO) from a heavy residue stream, the short residue (SR). Typically, the total feed to a HVU is first sent to a furnace and from there it goes to a high vacuum column where in some cases also stripping steam is used to further enhance the separation. The use of steam also has some disadvantages such as: a higher condenser duty, increased sour water production, a larger ejector set and a requirement for a larger column diameter due to additional (steam) vapor flow.
When aiming at maximizing the VGO recovery, this can be done by maximizing the operating temperature, by using stripping steam and/or by lowering the pressure in the vacuum distillation as much as possible.
It was now found that the disadvantages of steam mentioned above are overcome by using a preflash vessel upstream of the HVU. In such configuration, the feed is ‘flashed’ at mild vacuum conditions in the preflash vessel before entering the furnace of the HVU. The vapor flashed off is used for stripping the residue product in the HVU column, and as a result less (or no steam) is required.
In a refinery, a preflash vessel typically is known for its use in a CDU, but not in a HVU.
EP-A-0095792 relates to a flash distillation process wherein a reduced crude or long residue is passed to a flash tower after which the bottoms fraction obtained is stripped in a stripping tower. This bottoms fraction is introduced in the upper part of the stripping tower and flows downwards while in contact with steam and at most a limited amount of hydrocarbon vapor, if any. In this set-up, the vapor or gas added to the stripping tower mainly is steam. Furthermore, stripping is fundamentally different from distillation as stripping aims at preferentially removing light components while distillation separates a hydrocarbon stream into various fractions.